This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A386990 #22 Aug 19 2025 10:43:54
%S A386990 2,3,8,4,4,2,7,3,8,7,9,7,1,4,2,8,8,2,1,1,6,4,4,8,0,4,9,2,3,8,0,4,4,8,
%T A386990 1,8,4,6,1,4,9,8,5,7,0,6,4,6,6,9,8,7,8,4,8,4,1,7,2,0,3,9,5,2,0,8,9,0,
%U A386990 0,3,8,3,7,7,6,3,0,4,4,7,1,1,5,3,9,1,3,2,1,6,2,4,2,6,7,8,5,5,9,3,9,6,9,5,2,3
%N A386990 Decimal expansion of Sum_{k>=0} 2/(k!*(k! + 1)).
%C A386990 Sum of reciprocals of A055555 (triangular numbers of factorials).
%F A386990 Equals Sum_{k>=0} 1/A055555(k).
%e A386990 2.3844273879714288211644804923804481846149857064...
%p A386990 evalf(sum(2/(n!*(n!+1)),n=0..infinity), 120); # _Alois P. Heinz_, Aug 13 2025
%o A386990 (PARI) suminf(k=0, 2/(k!*(k!+1)))
%o A386990 (PARI) sumpos(k=0,1/binomial(k!+1,2)) \\ _Charles R Greathouse IV_, Aug 19 2025
%Y A386990 Cf. A000217, A070910 (of n!^2), A055555, A091131 (of n!).
%K A386990 nonn,cons
%O A386990 1,1
%A A386990 _Kelvin Voskuijl_, Aug 12 2025